The testing of customer loops by the central switch in modern digital telephone switching systems, such as the 5ESS Switch manufactured by Lucent Technologies Inc., is performed on both an analog and digital basis. Digital testing alone does not provide sufficient fault coverage of line card circuitry, and therefore, the performance of additional tests requires the direct connection of a test unit to check the electrical parameters of the subscriber loop. Digital testing is disclosed in Marsh et al, U.S. Pat. No. 5,596,322 issued Jan. 21, 1997 which describes a 16-channel programmable codec known as the T7531/35 manufactured by Lucent Technologies Inc. The Marsh patent employs the codec's DSP to calibrate or trim the amplifier gains of 14 of the channels based on the factory calibration of only two of the channels. In the Marsh method, the DSP applies analog calibration signals to each of the A/D paths and compares the output signals received from the paths with the output signal received from the pre-trimmed path to determine the A/D gain correction coefficients for the paths. The DSP also applies a digital calibration signal to each D/A path and sets up a loop-back condition for routing the output signal from the D/A paths to one of the A/D paths and then compares the results to set the D/A coefficients.
Another digital testing technique is disclosed in Lopresti et al U.S. Pat. No. 5,559,440 issued Sep. 24, 1996 and assigned to Lucent Technologies Inc. The Lopresti patent describes tests of the transmit and receive paths that traverse the line circuits by temporarily plugging in a personal computer to the line circuit's PCM bus to ascertain whether the A/ID and D/A channel gains, return loss, terminal balance return loss, noise and distortion parameters are within acceptable limits.
While the Marsh and Lopresti arrangement operate satisfactorily, the need exists for an automated, self-testing capability on the line card that does not require the connection of a personal computer to perform floating point calculations or to generate and detect the test tones. In addition, it would be desirable to perform certain tests on the line circuits, such as DC overhead measurement and peak amplitude measurement that cannot be performed by a central switching facility linked to the line circuits by a PCM bus and which introduces its attendant complications of companding, filtering, limited data sampling rate and the inability to convey DC signals.
While it might appear to be desirable to perform such tests locally at the line card and thereby eliminate the effects of the PCM bus, the implementation of such tests at the line card presents several problems. Among the problems presented are how to implement with limited processing capability the necessary digital tone generation and detection, how to compute the variance of signals having different DC offsets, how to accomplish peak detection all without substantially interfering with voice processing.